Bedrock Exposure Research Articles

Upper slope landforms and age of bedrock exposures in the St. Francois Mountains, Missouri: A comparison to relict periglacial features in the Appalachian Plateau of West Virginia

We observed upper slope landforms in the St. Francois Mountains of Missouri and compared them to relict periglacial landforms in the Appalachian Plateau of West Virginia in order to determine if periglacial conditions existed in southeastern Missouri. The St. Francois Mountains are the exposed igneous core of the Ozark Plateaus and are about 50 km from the Illinoian glacial boundary in southern Illinois and 75 km from the pre-Illinoian glacial boundary in east central Missouri. Summits of rhyolitic knobs have the appearance of cryoplanation summits but are structurally controlled, stratified pyroclastics with no evidence of periglacial features. Most rhyolite summits have a discontinuous mantle of 30–70 cm of loess. No patterned ground was observed in the St. Francois Mountains, but was observed at a summit in West Virginia. Block fields and block streams in Missouri are forested matrix-supported diamictons of large blocks partially buried in loess fines. Appalachian block fields and block streams largely consist of contiguous blocks with little or no fines and no vegetation. We measured length, width, aspect, slope, and size of the largest blocks along the axis of the Missouri block streams and block fields and on two typical West Virginia block streams (one 36Cl dated 143 ka, late Illinoian). The Missouri block accumulations show no size distribution patterns, are generally steeper and closer to the block source outcrop than the West Virginia block streams, and do not have the morphology of Appalachian relict periglacial landforms. Bedrock exposures in the St. Francois Mountains are old enough to have been exposed to any periglacial environments during the Pleistocene. One rhyolite summit has a 36Cl age of 536 ka (pre-Illinoian) at zero erosion. A granite summit has a 36Cl age of 333 ka (pre-Illinoian) at zero erosion.

Relief-rejuvenation and topographic length scales in a fluvial drainage basin, Napf area, Central Switzerland

This paper explores how, and to what extent, a phase of relief-rejuvenation modifies the mode of surface erosion in an approximately 63 km 2 drainage basin located at the northern border of the Swiss Alps (Luzern area). In the study area, the retreat of the Alpine glaciers at the end of the Last Glacial Maximum (LGM) caused base level to lower by approximately 80 m. The fluvial system adapted to the lowered base level by headward erosion. This is indicated by knickzones in the longitudinal stream profiles and by the continuous upstream narrowing of the width of the valley floor towards these knickzones. In the headwaters above these knickzones, processes are still to a significant extent controlled by the higher base level of the LGM. There, frequent exposure of bedrock in channels and especially on hillslopes implies that sediment flux is to a large extent limited by weathering rates. In the knickzones, however, exposure of bedrock in channels implies that sediment flux is supply-limited, and that erosion rates are controlled by stream power. The morphometric analysis reveals the existence of length scales in the topography that result from distinct geomorphic processes. Along the tributaries where the upstream sizes of the drainage basins exceed 100,000–200,000 m 2, the mode of sediment transport and erosion changes from predominantly hillslope processes (i.e., landsliding, creep of regolith, rock avalanches and to some extent debris flows) to processes in channels (fluvial processes and debris flows). This length scale reflects the minimum size of the contributing area for channelized processes to take over in the geomorphic development (i.e., threshold size of drainage basin). This threshold size depends on the ratio between production rates of sediment on hillslopes, and export rates of sediment by processes in channels. Consequently, in the headwaters, erosion rates and sediment flux, and hence landscape evolution rates, are to a large extent limited by weathering processes. In contrast, in the lower portion of the drainage basin that adjusts to the lowered base-level, rates of channelized erosion and relief formation are controlled mainly by stream power. Hence, this paper shows that base-level lowering, headward erosion and establishment of knickzones separate drainage basins in two segments with different controls on rates of surface erosion, sediment flux and relief formation.

Structure, detrital zircon U – Pb ages and 40Ar/39Ar geochronology of the Early Palaeozoic Girilambone Group, central New South Wales: subduction, contraction and extension associated with the Benambran Orogeny

The Girilambone Zone is located in the northern part of the central subprovince of the Lachlan Fold Belt and its bedrock exposure includes the widespread Girilambone Group. This unit consists of two main components, low-grade, metamorphosed quartz-rich turbidites and bedded chert with Middle to Late Ordovician conodont ages in the west (Ballast Formation), and low to moderate-grade schist, psammitic schist, mafic schist and phyllite that occur in the east. Detrital zircon grains from samples of a quartzite and a quartzose psammite from the eastern part of the Girilambone Group around Girilambone, have age spectra (600 – 500 Ma) typical of Ordovician turbidite successions in southeastern Australia. In the Girilambone district, an intense, steeply dipping differentiated layering (S2) formed in the D2 deformation and has largely masked the effects of earlier deformation apart from foliation (S1). S2 is overprinted by mainly north-trending, moderately to steeply west-dipping crenulation cleavage (S3) that is axial planar to abundant open to close folds (D3 deformation). Locally, S3 cleavage and fold axial planes have a low dip. In the Tottenham district, the main foliation is also a differentiated layering (S2) with low to moderate dips to the southeast in a northern domain and subhorizontal in a larger southern domain due to folding by abundant near-isoclinal, recumbent folds (F3). These reflect ductile thinning of the crust and are attributed to extensional deformation. Weak to locally intense crenulation cleavage and associated folds (D4) affect the S2 in the northern domain and are weakly developed in the southern domain. Three muscovite 40Ar/39Ar ages from both the Girilambone and Tottenham regions are ca 435 Ma consistent with deformation and metamorphism during the Late Ordovician – Early Silurian Benambran Orogeny. The Girilambone Group in the Late Ordovician was accreted in an inferred subduction complex related to an east-dipping subduction zone in the Wagga marginal sea. The subduction complex in the Early Silurian was affected by contractional and extensional deformation prior to the migration of igneous activity from the Molong island arc to the Wagga – Omeo Zone.

Field measurements of incision rates following bedrock exposure: Implications for process controls on the long profiles of valleys cut by rivers and debris flows

Until recently, published rates of incision of bedrock valleys came from indirect dating of incised surfaces. A small but growing literature based on direct measurement reports short-term bedrock lowering at geologically unsustainable rates. We report observations of bedrock lowering from erosion pins monitored over 1–7 yr in 10 valleys that cut indurated volcanic and sedimentary rocks in Washington, Oregon, California, and Taiwan. Most of these channels have historically been stripped of sediment. Their bedrock is exposed to bed-load abrasion, plucking, and seasonal wetting and drying that comminutes hard, intact rock into plates or equant fragments that are removed by higher fl ows. Consequent incision rates are proportional to the square of rock tensile strength, in agreement with experimental results of others. Measured rates up to centimeters per year far exceed regional long-term erosionrate estimates, even for apparently minor sediment-transport rates. Cultural artifacts on adjoining strath terraces in Washington and Taiwan indicate at least several decades of lowering at these extreme rates. Lacking sediment cover, lithologies at these sites lower at rates that far exceed long-term rock-uplift rates. This rate disparity makes it unlikely that the long profi les of these rivers are directly adjusted to either bedrock hardness or rock-uplift rate in the manner predicted by the stream power law, despite the observation that their profi les are well fi t by power-law plots of drainage area vs. slope. We hypothesize that the threshold of motion of a thin sediment mantle, rather than bedrock hardness or rock-uplift rate, controls channel slope in weak bedrock lithologies with tensile strengths below ~3–5 MPa. To illustrate this hypothesis and to provide an alternative interpretation for power-law plots of area vs. slope, we combine Shields’ threshold transport concept with measured hydraulic relationships and downstream fi ning rates. In contrast to fl uvial reaches, none of the hundreds of erosion pins we installed in steep valleys recently scoured to bedrock by debris fl ows indicate any postevent fl uvial lowering. These results are consistent with episodic debris fl ows as the primary agent of bedrock lowering in the steepest parts of the channel network above ~0.03–0.10 slope.

Sand composition in an Iberian passive-margin fluvial course: the Tajo River

The Tajo River, the 10th largest river in Europe, drains part of the western passive margin of Europe that includes multiple tectonic elements of the Iberian plate. Modern fluvial sand composition in the Tajo River drainage basin reflects the nature of the source region, which lies in the central part of the Iberian Peninsula. Four fluvial petrographic provinces (A, B, C and D) can be established in the Tajo drainage basin, corresponding well with the four principal structural units drained: (1) the Iberian Range; (2) the Hesperian massif; (3) The Tertiary Tajo basin; and (4) the Neogene Santarem–Lisboa basin. Province A corresponds to the Tajo River head and is characterized by quartzolithic sedimenticlastic sands (Qm 67F 4Lt 29 and Rs 79Rg 3Rm 18). These sands have been derived from diverse Mesozoic siliciclastics and carbonates of the Iberian Range. Province B appears in the upper reaches of the Tajo River course and is quartzofeldspathic (Qm 57F 34Lt 9) with diverse rock fragments (Rs 34Rg 23Rm 43). Sources are Hercynian granitoids and metasediments and Neogene clastics and minor carbonates of the Tertiary Tajo basin. Province C extends along the middle course of the Tajo River with quartzofeldspathic metamorphiclastic sand modes (Qm 60F 33Lt 7; Rs 4Rg 21Rm 75). The sources are metamorphic rocks intruded by plutonites of the Hesperian Massif. Province D is quartzofeldspathic (Qm 55F 39Lt 6) with a dominance of phaneritic rock fragments (Rs 5Rg 33Rm 62) and corresponds to the lower reaches of the Tajo River, where siliciclastic deposits of the Neogene Santarem–Lisboa basin are the main sources. Sands plot on provenance-discrimination diagram (QmFLt) within the recycled-orogen field (Tajo River head) and continental-block fields (upper, middle and lower course). In addition, we have proven the usefulness of the RsRgRm diagram to discriminate the defined fluvial provinces, originating from heterogeneous parent-rock textures and mineralogy. Climate does not exert any strong influence on the petrogenesis of the Tajo River drainage basin sand, and erosion in the source areas may be described in terms of weathering-limited denudation regime. By contrast, mixing with tributary supplies is the main process that modifies composition in the Tajo River sand. The establishment of fluvial provinces related to the main bedrock structural units reflects the great relevance of tributaries from each province in the generation of the Tajo River sand and the low significance of inherited sandy load from previous provinces. The abundance of granitoid rock fragments (RsRgRm%Rg) in fluvial sand of both tributaries and main channel of the Tajo River drainage basin faithfully represents the relative abundance of granodiorite+monzogranite bedrock exposure in each subbasin. Metasedimentary and metamorphic bedrock outcrop area is overrepresented by metamorphic rock fragments (RsRgRm%Rm) in both sand from tributaries and from the Tajo main trunk river. This is manifested by differences of 18–23% between means of metasedimentary outcrop area and the RsRgRm%Rm mean in the sand from Provinces B and C+D, respectively. Sedimentary (mainly carbonate) outcrop area is underrepresented or not represented by sedimentary rock fragments (RsRgRm%Rs) in sand of the Tajo River. This underrepresentation causes a difference between means of sedimentary outcrop area and the RsRgRm%Rs mean of 21%, 17.1% and 10.7% in sands from Province A, B and C+D, respectively. This fact confirms the rapid loss of these grains during transport because of their labile nature.

A comparison of electrical and electromagnetic methods for the detection of hydraulic pathways in a fractured rock aquifer, Clare Valley, South Australia

Within fractured rock, the irregular and often unpredictable distribution and geometry of hydraulically conductive fractures produces large spatial variations in bore yield and groundwater quality. As fractures act as conduits for flow of both groundwater and electrical charge, methods which can efficiently detect the distribution of electrical pathways can be used to infer characteristics of significant hydrological parameters. This study compares the capabilities and limitations of electrical data obtained from direct current (DC) and electromagnetic (EM) surface azimuthal measurements, and from DC borehole-to-surface and cross-borehole measurements, for the interpretation of major hydrological structures in Clare Valley, South Australia. Electrical and EM surface methods are limited by poor depth sensitivity and the presence of conductive overburden, but provide useful tools for determining directional variations in resistivity at sites lacking bedrock exposure and boreholes. Application of borehole-to-surface methods yielded a better-resolved interpretation of sub-vertical fracture strike and was useful in identifying lateral variations in bedrock heterogeneity. Improved flexibility and sensitivity to measurements at depth permitted cross-borehole electrical tomography data to be used in reconstructing the spatial distribution of sub-horizontal, laterally extensive, electrically conductive zones. While the technique is restricted to small-scale sites with multiple boreholes, inferences can be made on fluid connections over a much larger regional scale. It is important to note, however, that while electrical methods provide valuable information about in-situ hydraulic pathways, they do not provide a complete hydraulic characterisation. Such a task requires integration of surface and borehole geophysics, geologic mapping, sampling and pumping tests of wells with packed-off intervals.

Channel geometry of mountain streams in New Zealand

A dataset of 21 study reaches in the Porter and Kowai rivers (eastern side of the South Island), and 13 study reaches in Camp Creek and adjacent catchments (western side of the South Island) was used to examine downstream hydraulic geometry of mountain streams in New Zealand. Streams in the eastern and western regions both exhibit well-developed downstream hydraulic geometry, as indicated by strong correlations between channel top width, bankfull depth, mean velocity, and bankfull discharge. Exponents for the hydraulic geometry relations are similar to average values for rivers worldwide. Factors such as colluvial sediment input to the channels, colluvial processes along the channels, tectonic uplift, and discontinuous bedrock exposure along the channels might be expected to complicate adjustment of channel geometry to downstream increases in discharge. The presence of well-developed downstream hydraulic geometry relations despite these complicating factors is interpreted to indicate that the ratio of hydraulic driving forces to substrate resisting forces is sufficiently large to permit channel adjustment to relatively frequent discharges.

‘Keeping the boys busy’:1 Outdoor theatres of the Great Depression: On-site, incremental design gives form to the complex relationship of site and structure

In most cases today, every detail of new construction in public landscapes in the United States, from the location of walls to the joints in pavement and the size of bolts, is drawn and specified on computer screen or paper before ground is broken. Building contractors then construct the project fron1 the designer's detailed drawings. During construction, the designer's role is limited to attending periodic job meetings where they attempt to insure that their original intentions are realised.2 This separation of design and construction is a twentieth-century phenomenon. Throughout history, designed landscapes have been created through an incremental and interactive process of design, construction, observation and re-design. As construction progressed, both the partially completed improvements and the emerging intricacies of a site, such as an unexpected exposure of bedrock or a newly revealed vista, informed designers how to craft the relationship between the site and the design intervention. Variations of this process underlie our most influential landscape designs, from the great gardens of Italy, France and England to more recent icons such as Central Park and Dumbarton Oaks.3

Drumlin formation time: evidence from northern and central sweden

Large‐scale drumlins occur abundantly throughout central and northern Sweden. Whereas many drumlins in the north are an integral part of a relict glacial landscape >100,000 years old, those to the south are generally interpreted as of last deglaciation age. Typically, the latter ones have not been overprinted by younger glacial landforms. Despite this apparent difference in formation history, drumlins in both regions have similar directional and morphological characteristics. A systematic analysis of >3000 drumlins in (i) areas within relict landscapes, (ii) areas with an ambiguous deglaciation age assignment, and (iii) areas within deglacial landscapes, indicates that these latter deglaciation drumlins differ clearly in both shape and size from drumlins in the other two types of landscapes. In addition, numerical modelling indicates that basal melting conditions, a prerequisite for drumlin formation, prevailed only for a very limited time over much of northern Sweden during the last deglaciation, but lasted for longer periods of time during earlier stages of the Weichselian. A reconnaissance radionuclide bedrock exposure date from the crag of a large drumlin in the relict landscape indicates that glacial erosion, and presumably drumlin formation, at this location predated Marine Isotope Stage 7. We conclude, therefore, that the large‐scale drumlins of central and northern Sweden did not form during the last deglaciation, or during any other specific ice flow event. Instead, we suggest that they were formed by successive phases of erosion and deposition by ice sheets of similar magnitude and configuration.

Recent Changes in Areal Extent of the Devon Ice Cap, Nunavut, Canada

Image data from the years 1959/1960 and 1999/2000 reveal a 2.4% decrease in the surface area of the Devon Ice Cap, Nunavut, over the last 40 yr. This has resulted primarily from extensive retreat of tidewater glacier margins on the eastern side of the ice cap, and shrinkage of its near-stagnant southwestern arm. Thinning of the ice cap has also increased bedrock exposure in the ice cap interior. However, since 1960 the northwestern margin of the ice cap has advanced slightly. Volume loss associated with these changes was estimated at −67 ± 12 km3 as calculated from two independent techniques. A digital elevation model (DEM) of the ice cap surface was used to delineate interior ice divides allowing patterns of change to be investigated at the drainage basin scale. Strong correlation between the hypsometric characteristics of drainage basins and the observed changes in ice-cap geometry suggests that these changes reflect interbasin differences in the inherent sensitivity of glacier mass balance to recent climate forcing. Response time calculations indicate that most of the ice cap is responding to recent climate warming, whereas the northwestern region is likely still responding to cooler conditions that prevailed during the Little Ice Age (LIA).

Bed sediment sources and mixing in the glacierized upper Fraser River watershed, east‐central British Columbia

AbstractThe geochemical, mineralogical and lithological composition of modern stream bed material is examined in order to characterize sources and evaluate downstream mixing of sediments in the upper Fraser River drainage basin, British Columbia. The <63 µm fraction is emphasized for its relative mobility and ease of analysis using instrumental neutron activation. Overall, the composition of the stream sediments closely reflects bedrock distribution. Samples dominated by limestone and dolostone, calcite and dolomite, and related elements (Ca, Mg, Sr etc.) correspond to Lower and Middle Cambrian carbonate bedrock largely confined to the Moose River sub‐basin. Clastic and non‐quartzite metamorphic lithologies, primary and secondary aluminosilicate minerals and related elements (Al, Cs, Rb etc.) are largely derived from Miette Group bedrock and associated with the uppermost Fraser River sub‐basin. Except in the case of the Moose River/Fraser River junction, the determination of proportional tributary contributions is complicated by variable or delayed mixing, localized floodplain or valley side sources, and limited contrast between source areas. At present the Moose River sub‐basin contributes a greater proportion of the total and fine‐grained sediment loads of the combined Fraser River than would be expected from drainage basin area alone. The imbalance is related to greater relief, precipitation and runoff in the Moose River sub‐basin; however, the spatial association of carbonate‐rich stream sediments, ice cover and carbonate bedrock exposure indicates that glaciers play a particularly important roll in generating fine‐grained fluvial sediment. Since differences in glacier cover and glacier potential in the two major sub‐basins are likely to be persistent, and since relative sediment yields from the sub‐basins can be determined from sediment composition, a potential indicator of glacier variation and climate change during the Holocene is therein available. Copyright © 2004 John Wiley & Sons, Ltd.

The role of subsurface water flow paths on hillslope hydrological processes, landslides and landform development in steep mountains of Japan

AbstractHydrological monitoring was conducted in high‐relief watersheds in the Japan Alps to investigate the relationship between hillslope hydrological processes and landform evolution in steep granite and shale mountains. In the Koshibu watershed, underlain by Mesozoic shale, the drainage density and frequency was significantly lower than in the Yotagiri watershed underlain by granite. Drainage micro‐morphology analysis showed that hillslopes in the watersheds K1 and K6 (Koshibu basin) are mostly combinations of talus and bedrock exposures. In contrast, watershed Y1 (Yotagiri basin) is composed of several zero‐order streams with hollows. Infinite slope stability analysis indicates that the regolith shear strength in the K6 watershed (Koshibu basin) is lower than that of the Y1 hillslope, but groundwater levels were higher in the Y1 hillslope than in the K6 hillslope during storm events. These data suggest that, although the shear strength of the soil is stronger in the Yotagiri watershed, the slopes are unstable because of the groundwater conditions, whereas deep‐seated landslides may occur episodically in the Koshibu watershed associated with extreme storms and very high antecedent soil moisture. These differences would strongly contribute to the different observed hillslope processes and drainage characteristics. Copyright © 2004 John Wiley & Sons, Ltd.

Observations on weathering forms at the Caniapiscau Reservoir, North‐Central Québec, Canada

Weathering forms were encountered during field work in a formerly glaciated shield area in Canada that lies within the Labrador‐Ungava sector of the former Laurentide Ice Sheet. The granite and gneissic granite in the study area display features of surface weathering, such as weathering pits, and of deep weathering, such as an exposed weathering front with associated gravelly saprolite and core stones. Most observations were made along the shoreline of the Caniapiscau reservoir, where wave abrasion since the construction of a hydropower dam has lead to recent exposure of bedrock over extensive areas. The saprolite remnants emphasise the importance of weathering processes in shaping of bedrock relief in the area and together with the other weathering forms indicate restricted glacial erosion. The weathering forms are discussed in the context of glacial erosion and preservation mechanisms.

Unique human-made catenary changes and their effect on soil and vegetation in the semi-arid Mediterranean zone: a case study on Sarcopoterium spinosum distribution near Tell eṡ-Ṡâfi/Gath, Israel

The semi-arid Mediterranean landscape in the area of Tell eṡ-Ṡâfi (biblical Gath of the Philistines), in central Israel, has been affected by extraordinary human activities. The Judean foothills in the region are characterised by chalk and calcrete with a patchy cover of Rendzina soils. Unique human-made geomorphic and catenary changes in the landscape, nearly 3000 years ago, left their marks on the vegetation distribution in the area today. A very large trench that surrounds the tell on three sides was excavated in the Iron Age II (ca. 9th century B.C.E.). The trench served as a siege moat, part of an offensive siege system around the tell. The trench is 2.5 km long, 8–10 m wide, and has a maximum depth of 5.5 m. A berm, consisting of dumped earth and rock fragments excavated from the trench, is situated alongside the entire length of the trench, covering the original surface. Our catenary landscape analysis revealed that a particular vegetation species, Sarcopoterium spinosum, is dominant on the berm, covering 78% of its surface area. Its coverage of the three other catenary landscape units, slope above the berm, trench, slope below the trench, is only 29%, 32% and 24%, respectively. Bedrock exposure in the berm is less than 4%, which together with the slightly elevated position of the berm results in surface area conditions of relatively low water retention. Such areas are conducive to S. spinosum establishment and survival, due to the plant's high resistance to comparatively dry conditions. Thus, the human landscape changes almost three thousand years ago caused a dramatic catenary change that continues to have a distinct effect on vegetation distribution today.

A Comparison of Electrical Methods for Detection of Hydraulic Pathways in a Fractured Rock Aquifer, Clare Valley, South Australia

Within fractured rock, the irregular and often unpredictable distribution and geometry of hydraulically conductive fractures produces large spatial variations in bore yield and groundwater quality. As fractures act as conduits for flow of both groundwater and electrical charge, methods that can efficiently detect the distribution of electrical pathways can be used to infer characteristics of significant hydrological parameters. This study compares the capabilities and limitations of electrical data obtained from direct current (DC) and electromagnetic (EM) surface azimuthal measurements, and from DC borehole-to-surface and cross-borehole measurements, for the interpretation of major hydrological structures. Data obtained from surface methods, while limited by poor depth sensitivity and overburden provides a useful tool for determining variations in directional conductivity at sites free of bedrock exposure and boreholes. Application of borehole-to-surface methods yielded a better resolved interpretation of sub-vertical fracture strike and was useful in identifying lateral variations in bedrock heterogeneity. Improved flexibility and sensitivity to measurements at depth permitted cross-borehole electrical tomography data to be used in accurately reconstructing the spatial distribution of sub-horizontal, laterally extensive, electrically conductive zones. While the technique was restricted to sites with multiple boreholes, the data obtained was used to constrain specific flow paths over a regional scale.

Review of the Ordovician rhynchonelliformean Brachiopoda of the East Baltic: their distribution and biofacies

Analyses of the distribution, in time and space, of approximately 300 Ordovician rhynchonelliformean brachiopods in the East Baltic allow the development of a faunal template for the Baltic Province (sensu stricto) within the context of the European Realm. Two different brachiopod magnafacies, the upper and lower ramp associations, are monitored through time. Changes in the brachiopod fauna through uppermost Hunneberg to the Porkuni stages are demonstrated from different drill core sections and some bedrock exposures located in facially contrasting areas across the region. The main developmental trends within the brachiopod biofacies of the shallower part of the palaeobasin (North Estonian facies belt) are characterized by relatively continuous changes in taxonomic composition including the evolution of endemics and the establishment of relatively persistent associations, especially during the later Ordovician. In the deeper parts of the palaeobasin (Central Baltoscandian confacies belt including the Livonian Tongue) the several different types, clearly determined by changes in environment, occur: Relatively low diversity associations in the red-coloured sediments, a well-defined assemblage associated with black shales and more diverse associations in the argillaceous carbonate deposits. The appearance and distribution of some shortlived associations including immigrants to the Baltic (Dactylogonia and Rhynchotrema during the Keila-Oandu event, the Holorhynchus association during the mid-Ashgill and the Hirnantia fauna during the late Ashgill) are probably associated with climatic and sea-level changes in the palaeobasin.

Quantitative bedrock geology of Alaska and Canada

We quantitatively analyze the area‐age distribution of sedimentary, igneous, and metamorphic bedrock on the basis of data from the most recent geologic maps of Alaska [Beikman, 1980] (1:2,500,000) and Canada [Douglas, 1969] (updated; 1:5,000,000), made available in digital form by the U.S. Geologic Survey and the Geologic Survey of Canada (National Resources Canada), respectively. Sediments cover 72.9% and 52.4% of the surface of Alaska and Canada. Volcanic rocks comprise 11.6% and 6% of the surface area in Alaska and Canada, respectively, whereas intrusive rocks cover 6.6% and 24% of the surface, respectively. Ultramafic rocks account for 0.20% and 0.08% of the bedrock area, whereas metamorphic rocks cover 3.4% and 16.1%. The average ages of major lithologic units, weighted according to bedrock area, are significantly younger in Alaska than in Canada: marine sediments (stratigraphic age of 206 Myr in Alaska versus 599 Myr in Canada), continental sediments (stratigraphic age of 81 Myr versus 150 Myr), volcanic rocks (126 Myr versus 1377 Myr), intrusive rocks (114 Myr versus 2109 Myr), ultramafic rocks (261 Myr versus 874 Myr), and metamorphic rocks (474 Myr versus 2545 Myr). This age difference reflects the contrast between the young active margin influence on the geology of Alaska and the old cratonic nature of much of the bedrock exposed in Canada. Glacial erosion, and the resulting exposure of Precambrian bedrock, primarily in Canada, contributes to this age contrast. The average temporal and spatial resolution of the digital data is sufficiently high to perform age‐area analyses on individual river basins larger than ∼20,000 km2 and to evaluate the relationship between bedrock geology and river chemistry.

Growth of Calcareous Epilithic Mats in the Margin of Natural and Polluted Hydrosystems: Phosphorus Removal Implications in the C–111 Basin, Florida Everglades, USA

ABSTRACT The Florida Everglades is a fragile wetland system that is naturally depleted in phosphorus (P). This hydrosystem has been heavily impacted by human activities, including the draining of wetlands to provide water for agricultural and urban use. The result is a highly compartmentalized system with altered hydropatterns; wetlands receive canal discharges from diffuse agricultural/urban runoff containing high levels of pollutants, including P. Excess loading of P has induced ecological changes, including dramatic effects on periphyton, the dominant producer community. An Everglades rehabilitation plan has been established to restore natural hydropatterns and decrease P loads. On the southern edge of a large canal (C-111) draining the southern Everglades, levees have been removed to rehabilitate hydrology in the adjacent marsh. Levee removal resulted in exposure of limestone bedrock that, when flooded with shallow water from the canal, favors the development of thick calcareous epilithic mats. When flooded approximately 6 months a year, this margin area between the polluted and the natural hydrosystem functions as a Periphyton-based Stormwater Treatment Area(PSTA), a biological tool developed at the Southeast Environmental Research Center (Miami, FL), being considered in the Everglades as a means of P removal from enriched waters. Here, we evaluate the harvesting rates of periphyton that promote the most efficient removal of TP from the water entering the marsh. Results indicate that harvesting of periphyton at 3 month intervals provides the greatest TP sequestration.

An assessment of land use and other factors affecting sediment loads in the Rio Puerco watershed, New Mexico

Rapid channel erosion in the Rio Puerco watershed of northwest New Mexico has been attributed to land use, climate changes, and internal channel adjustments. The objectives of this study were to assess (1) the impacts of land uses on sediment load, (2) the quantitative relationships between land use and sediment load, and (3) the effectiveness of different erosion control methods. The impacts of land uses on sediment load were assessed via hypotheses that, holding other erosion-related variables constant, sediment load correlates positively with grazing intensity and with density of unpaved roads, and correlates negatively with the number of erosion control treatments. We calculated the average annual sediment load for 17 subbasins of 0.67–17.97 km 2 by comparing sediment accumulation at two points in time (mid-1960s and 1999) behind intact sediment retention structures. We assessed land use via grazing records and measurements of unpaved roads generated from aerial photographs. Soil characteristics, vegetation, and physical factors were quantified for each subbasin. Using 18 variables for each subbasin, we employed Mallow's C p as a selection criterion. We used six statistical models, including multiple regression and principal components analysis, to determine inherent mathematical relationships between significant independent variables and sediment load. The results indicate that sediment load does not correlate with grazing intensity except in small, relatively low-relief basins with fewer bedrock exposures. However, this interpretation may be compromised by the low quality of data available to quantify grazing. Sediment load is highly sensitive to the presence of unpaved roads, which serve as high gradient, channelized conduits of water and sediment during storms. Sediment load does not correlate with erosion control except in the subset of small, relatively low-relief subbasins that also proved sensitive to grazing intensity. Overall, the statistical analyses suggest that the impact of land uses on sediment load can be assessed using the data available although physical factors appear more significant than land use in producing high sediment loads in the Rio Puerco subbasins. Historical erosion control techniques, as implemented in the Rio Puerco watershed, prove largely ineffective against accelerated sediment loads because they have not been appropriately implemented or maintained.

Origin of vertical shafts in bedrock along the Eramosa River valley near Guelph, southern Ontario

Numerous vertical to subvertical, cylindrical shafts occur in the rugged exposures of the Middle Silurian Amabel Formation bedrock along the Eramosa River, approximately 10 km northeast of Guelph. These shafts vary from < 1 m to 10 m in diameter, with depths as great as 12 m. They can occur as isolated depressions but more commonly are clustered closely together, and many coalesce to form sinuous bedrock walls. These depressions may contain soil, fine sediment, rubble from the surrounding dolomite bedrock, and rounded clasts of Precambrian basement lithologies. Till is conspicuously absent. These shafts occur in close proximity to cavernous pores that were formed during pre-Wisconsinan to Early Wisconsinan time, suggesting a possible karstic origin. The cylindrical shafts, however, with their greater midpoint diameters, sculpted interiors, and inclined axes, and their lack of vertical flutes and till, are more likely to have originated from glaciofluvial discharge at the end of the Late Wisconsinan. The development of these shafts may have been assisted by cavitation, which enhanced the erosional capacity of sediment-laden meltwaters.